This invention relates in general to the field of digital receivers, in particular to the decoding of speech signals and more particularly to the improvement in audio quality by the detection of channel errors.
With the emergence of new digital cellular-type telephones into the high volume commercial marketplace, voice compression algorithms are becoming commonplace. Due to the nature of voice coders and decoders, (i.e., vocoders) channel errors typically induce unusually offensive artifacts into a decoded speech signal. This is especially true when the spectral components of the speech signal becomes corrupted.
Line spectral pairs (LSPs) are typically used in modern vocoders because of their perceptual qualities and because LSPs are typically very well behaved. These characteristics allow for efficient coding and compression of the spectral content of a speech signal before its transmission across narrow band communication channels. The spectral content of voice signals is typically slowly evolving or changing. However, when a channel error corrupts an LSP parameter, it will usually cause dramatic and excessive changes in the spectral content of the signal. As a result, high-energy chirps or squawks are provided in the decoded signal which may be very offensive sounding.
In another example where digital voice information is encrypted, the receiver""s loss of cryptographic synchronization results in improperly decrypted speech signals. The speech decoder in this case typically also provides offensive high-energy chirps and squawks until cryptographic synchronization is re-established.
Accordingly, what is needed are an apparatus and method for detecting offensive spectral errors. What is also needed are a method and apparatus for correcting offensive spectral errors. What is also needed are a method and apparatus that detects and corrects offensive spectral errors which result due to channel errors or the loss of crypto-synchronization.